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Old 10-15-2011, 03:46 PM   #1
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Furnace continues to run after system shuts down


Hello,

I think there might be something wrong with my heat/ac system. I had it on COOL for the summer and today I went to the thermostat and switched it from COOL to OFF.

The heat pump shut off and the cold air stopped but the furnace kept going. I left it alone for a while to see if it was just the fan but it never shut off.

So, I tried to do a full reset by turning off the breakers to the furnace and heat pump, which in turn killed the power to the thermostat.

I left everything off for 5 or 10 minutes and turned it all back on, heat pump first, then furnace.

When I did that I checked the thermostat again and it was set how I left it, the system is OFF and fan is set to AUTO.

At this point I heard the furnace come on for about 8 or 9 seconds, then go off for a few seconds, then come back on for 8 or 9 seconds. It did this what seemed like 4 or 5 times.

Then the furnace came on and just stayed on. Right now it's been running for about 30 mins, the air coming out of a vent I checked feels warm.

Does anyone know what I can check to try to determine why it's acting like this?

The furnace is an International Comfort
model NFCP2400D1
ABF2400D1

Style number: FCP2400D

I took off the panel and I don't see any relays. There is a circuit board on the top left side that looks like it has everything going through it.

There are a few other connections in the very back, past the circuit board, one looks like a plastic computer power supply connection and I can't really see the other one without moving things around.

Of course, I killed the power first.

Could my circuit board be bad? Do they sell those at lowes?

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Old 10-15-2011, 03:47 PM   #2
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Furnace continues to run after system shuts down


The circuit board I mentioned has this number on it:

HQ1085914TX

If that helps at all....

Also, in an attempt to rule out the thermostat, I shutdown the power to everything and popped the thermostat off of the wall.

Then I returned the power and after maybe 5 or 10 seconds of standing in front of the furnace I heard a click sound and it powered on, again for only a few seconds, then shutdown for a few seconds, back on for a few seconds, etc.

I believe that means the message to startup isn't coming from the thermostat. I think.

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Old 10-16-2011, 04:19 AM   #3
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Furnace continues to run after system shuts down


Might have a short in a strip heater.
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Old 10-16-2011, 07:10 AM   #4
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Furnace continues to run after system shuts down


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Originally Posted by beenthere View Post
Might have a short in a strip heater.
Last night I pulled all of the thermostat wires from the furnace and powered it back on and the unit started back up.

Looking up the part number of the board showed it to be a fan control board so I figured that must be the problem. I ordered a new one and am waiting for it to come.
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Old 10-16-2011, 07:12 AM   #5
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Furnace continues to run after system shuts down


Seldom is the board the problem.
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Old 10-16-2011, 08:50 AM   #6
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Furnace continues to run after system shuts down


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Seldom is the board the problem.
So I just wasted $100? What can I do to narrow in on the problem please?
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Old 10-16-2011, 09:02 AM   #7
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Furnace continues to run after system shuts down


Last night on the phone I talked to an old friend that says he does hvac for 14 years. He told me to remove the thermostat wires from the furnace and turn the power back on.

When I restored the power, after about a minute the furnace started. Coming on for a few seconds and then shutting off. After doing that a bunch of times it then started and never stopped.

So, he told me to replace the board.

One thing to note, I have to double check this but I'm pretty sure the coils were not hot to the touch after running for a few mins.
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Old 10-16-2011, 11:38 AM   #8
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Furnace continues to run after system shuts down


Post brand and model number of furnace. Some have a high limit that plugs in by a molex plug, which can come loose and cause this.
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Old 10-16-2011, 11:44 AM   #9
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Furnace continues to run after system shuts down


Quote:
Originally Posted by beenthere View Post
Post brand and model number of furnace. Some have a high limit that plugs in by a molex plug, which can come loose and cause this.
See post 1

"The furnace is an International Comfort
model NFCP2400D1
ABF2400D1

Style number: FCP2400D"

Sounds like an ICP air handler.
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Old 10-16-2011, 11:53 AM   #10
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Furnace continues to run after system shuts down


Quote:
Originally Posted by hvactech126 View Post
See post 1

I saw it. i just missed that part. LOL... thanks.

"The furnace is an International Comfort
model NFCP2400D1
ABF2400D1

Style number: FCP2400D"

Sounds like an ICP air handler.
Check for loose electrical connects to the transformer, and to the board from the transformer.
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Old 10-16-2011, 12:21 PM   #11
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Furnace continues to run after system shuts down


Quote:
Originally Posted by beenthere View Post
Check for loose electrical connects to the transformer, and to the board from the transformer.
I pushed all the connections on the board and transfer down, none seemed loose except the big white plastic one on the board that I believe is what powers the board, it was out just a hair.

While I was at it, I test continuity on all of the thermostat wires and they are all good.

Edit: ...But, it's still doing the same thing.

Last edited by spikesnet; 10-16-2011 at 12:23 PM. Reason: Forgot the results
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Old 10-16-2011, 01:47 PM   #12
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Furnace continues to run after system shuts down


While it could be the board. Don't be surprised if the new board does the same thing.
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Old 10-16-2011, 01:54 PM   #13
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Furnace continues to run after system shuts down


Quote:
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While it could be the board. Don't be surprised if the new board does the same thing.
If the new board does the same thing, where do I go from there?
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Old 10-16-2011, 02:06 PM   #14
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Furnace continues to run after system shuts down


Bypass the high limit and see if it does it. Check boar for burnt resister.
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Old 10-16-2011, 02:10 PM   #15
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Furnace continues to run after system shuts down


I didn't see anything on the board that looks burnt. I'm not sure how to bypass the high limit....

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